The invention relates to a method and an arrangement for controlling the metering of fuel of internal combustion engines equipped with an exhaust-gas turbocharger wherein the inducted air mass is determined via an air mass measuring apparatus and wherein this air mass is the primary control quantity for the computation of the cylinder charge.
It is known to compute the fuel metering in turbo engines with an air mass measuring apparatus in the intake manifold. For this purpose, the air mass, which is inducted by the engine, is measured via an air mass measuring apparatus such as a hot wire air mass sensor (HLM) or a hot film air mass sensor (HFM). The air mass measured in this manner serves as a primary control quantity for the computation of the injection quantity in the injection system of the engine. A control apparatus of the injection system receives input signals from these sensors, namely, the air mass measuring apparatus, and computes the injection time as a measure for the fuel quantity to be injected from these input signals and additional stored data.
The technology of hot air mass sensors as air mass measuring apparatus has the disadvantage that a pressure drop in the intake manifold forward of the throttle flap cannot be detected. The air mass measuring apparatus (HFM or HLM) must be preset with imprecise triggering thresholds because of the following: the component tolerances of the hot air mass sensor, the component tolerances of the throttle flap potentiometer and of the throttle flap support as well as because of density changes in the inducted air because of different temperatures and ambient pressures. The occurrence of a leak in the intake manifold ahead of the throttle flap leads to the situation that the pressure drop is not detected by the air mass measuring apparatus and the fuel quantity is incorrectly computed and the engine can die because of the incorrect metering of fuel. In such a case, it is not wanted that the air mass measuring apparatus reacts to the pressure change because it is not the change of the air mass at the measuring apparatus which is the problem, but a leak ahead of the throttle flap. In the described method and control arrangements, a leak in the intake manifold therefore leads to an unwanted dying of the engine. A leak in the intake manifold of this kind can, for example, be caused by a bite of a marten or a loose hose clamp.
It is an object of the invention to provide a method and a control arrangement for the metering of fuel in turbo engines which make possible the detection of pressure drops in the intake manifold ahead of the throttle flap in each operating phase of the engine when a leak occurs. It is a further object of the invention to provide an effective emergency control of the metering of fuel when a leak occurs in the intake manifold.
The method of the invention is for controlling the metering of fuel of an injection system of an internal combustion engine having an intake manifold including an engine having an exhaust-gas turbocharger. The method includes the steps of: determining the inducted air mass (L) in the intake manifold of the engine as a primary control quantity utilizing an air mass measuring apparatus; computing a cylinder charge with a control apparatus; providing a substitute signal (P) as the primary control quantity; detecting a leak occurring in the intake manifold from a changed pressure ratio; and, switching over to the substitute signal (P) when the leak is detected.
With the method of the invention, a leak is detected in the intake manifold and, when such a fault is detected in the intake system, a switchover is made to a substitute signal as the primary control quantity for the metering of fuel. For this reason, a continued operation of the engine is ensured in every operating situation of the engine, that is, during charging operation as well as during induction operation of the exhaust-gas turbocharger. In this way, a stalling or dying of the engine because of an incorrectly measuring air mass measuring apparatus is avoided. With the occurrence of a fault in the intake system, a switchover takes place from the signal of the air mass measuring apparatus (HFM or HLM) to a computed substitute signal P. Occurring faults can be detected in all operating regions of the turbocharged engine. In the intake region of the engine, a leak between the compressor of the exhaust-gas turbocharger and the throttle flap can, for example, be detected. In the charge pressure region, that is, when the intake manifold pressure is greater than the ambient pressure, a pressure loss because of a leak can likewise be detected which, for example, is caused by a loose pipe clamp of the intake manifold. In both cases, an automatic switchover to a substitute signal takes place from the actual primary control signal of the air mass measuring apparatus and, in this way, an emergency operation is ensured notwithstanding the occurrence of the fault.
In accordance with an advantageous embodiment of the invention, the substitute signal is computed from the angular position of a throttle flap and the instantaneous engine rpm. The fuel metering thereby is based on an actual air mass, which is inducted by the engine, and not on the defective or incorrect air mass signals of the air mass measuring apparatus. The situation is avoided wherein the engine dies or stalls in idle or during load operation because of the fault. The fuel metering takes place without large deviations from the ideal value and, for this reason, larger lambda deviations and therefore deviations in the ideal mixture composition are avoided.
According to a further advantageous embodiment of the invention, a stored characteristic field with actual pressure conditions in the intake manifold is used to detect leaks in the intake manifold, that is, between the compressor of the exhaust-gas turbocharger and the throttle flap of the injection system. The pressure condition is, for example, the ratio of intake pipe pressure to charging pressure. In the case of a leak, the actual pressure ratio is greater than the value which is stored in the characteristic field for this pressure ratio. In this way, and in a simple manner, a leak in the intake manifold is detected because only an adaptation of the control apparatus and the storage of the characteristic field are required. The leak has the effect that, during idle operation of the engine as well as during load operation, the air mass flow measured by the air mass measuring apparatus is greater than the actual air mass flow inducted by the engine. From the comparison with the pressure ratios in the characteristic field, the control arrangement can detect a leak in the intake manifold.
According to an advantageous embodiment of the invention, a switchover to the substitute signal takes place only after the elapse of a preset delay time after the detection of a leak. In this way, the situation is avoided that already short-term deviations, which are not caused by a leak in the air intake system, are erroneously detected as a leak. With the method of the invention and the control arrangement, the situation is avoided that, in the case of a leak, an incorrect air mass is measured and, in this way, air masses which are too large or too small are processed in the control arrangement for computing the mixture formation in accordance with the particular operating state of the turbocharged engine. This would lead to the situation that a mixture is formed during fuel metering which is either too rich or too lean. Depending upon the size of the leak, such a deviation can no longer be controlled out by the lambda controller or by an adaptation of the mixture. Accordingly, with the invention, an uneven running of the engine or, in the worst case, a stalling or dying of the engine is avoided.